Conventional two-way communication systems separate the transmitted (Tx) and received (Rx) signals at each terminal in time, frequency, or space. Full duplex (FD) systems transmit and receive the radio frequency (RF) signal simultaneously. Most often this may be implemented by frequency division duplexing (FDD) where the Tx and Rx bands may be sufficiently separated in frequency such that filters can adequately attenuate any energy from the Tx signal that would leak into the Rx signal path and otherwise corrupt the Rx signal and prevent proper operation.
Frequency separation of the Tx and Rx bands apply in particular to mobile, portable, or nomadic devices where the antenna position cannot be precisely controlled, or the wavelength is on the order of, or larger, than the dimensions of the devices involved or the area available for antenna mounting.